The present invention relates to semiconductor integrated circuits and, more particularly, to an on-chip supply voltage regulator for regulating power supplied to a voltage controlled oscillator (VCO) in a phase-locked loop (PLL).
A certain class of VCOs are built from a series of inverting delay elements, with the output of each delay element connected to the input of the next delay element. When the number of inverting delay elements is odd and other conditions are met, the series of inverting delay elements will oscillate. The propagation delay through each delay element is typically set by the current available in the delay element to charge and to discharge the load present at its output. By controlling this current, the propagation delay and thus the frequency of oscillation is thereby controlled. The current available in each delay element is typically set by a voltage, hence the frequency of oscillation is voltage-controlled.
Voltage-controlled oscillators are used in a variety of applications, including PLLs. A PLL is constructed by combining a VCO in a closed-loop with a phase detector, a charge pump and a filter. The performance of the PLL regarding the spectral content of the signal at the VCO output is dependent on, among other things, the sensitivity of the VCO frequency to changes in power supply voltage. This parameter is commonly referred to as VCO power supply gain, and is specified in MHz/V. There is a continuing need for VCOs having a very low VCO power supply gain.